Embodiments of the invention relate to an electrical machine that can function as either a motor or a generator. The machine uses high frequency commutation of magnetic flux to achieve high efficiency and high power density.
Motors and alternators are designed for high efficiency, high power density, and low cost. High power density optionally is achieved by operating an alternator at high rotational speed and therefore high electrical frequency. However, high electrical frequency results in high core losses and lower efficiency. It would be desirable to provide a motor and alternator which had very low core losses thus making it practical to run it at high electrical frequency.
If a high rotational speed cannot be provided then the prior art motor or alternator must have a large number of poles to provide a high electrical frequency at low rotational speed. There is a practical limit to the number of poles a prior art motor or alternator can have due to space limitations, so once that limit is reached in order to reach a certain power level the motor or alternator must be relatively large and have the low power density inherent in low rotational speeds. It would be desirable to provide a motor or alternator that could have many times the number of poles currently possible providing high power density and good efficiency even at low rotational speed.
Another issue with prior art motors and alternators is that they require either a permanent magnet or an electromagnet to provide a magnetic field. Each type of magnet has some advantages and some disadvantages so that it is necessary to make a trade-off decision between the two types of magnets. Permanent magnets provide simplicity and they have the advantage that they do not require electrical input and thus allow a brushless motor or alternator. Permanent magnets also make it possible to design a motor or alternator of relatively high power density. However, they do not allow for operation over a wide speed range and they cannot be de-energized if desired. Electromagnets can be de-energized however they take up more space and require slip rings to draw power, power which is a parasitic loss for the system. Therefore, to avoid the need for a clutch, many machines must settle for a motor or alternator with lower power density, lower efficiency, and higher complexity. It would be desirable to provide a motor and alternator that could combine the advantages of permanent magnets and electromagnets.
The design trade-offs of existing motors and alternators have hindered commercial success of some motors and alternators. For instance, hub motors to drive the wheels of vehicles have not been commercialized because the low speed output requires a large motor that is not compatible with weight and size requirements of a vehicle suspension and drive system. A successful hub motor would require a power density that is many times higher than provided by prior art motors and it would have to maintain good efficiency and have variable field strength. Such a motor would go a long way toward making electric vehicles and hybrid-electric vehicles commercially acceptable.
Embodiments of the present invention provide a motor/alternator that provides a power density that is many times higher than prior art devices. This is achieved primarily by greatly reducing the core losses, allowing the motor/alternator to run at a much higher electrical frequency. Since it operates at high electrical frequency, at a given rotational speed, if the device is operated as an alternator, the output voltage is higher than for a prior art alternator. This reduces the current flowing through the windings and substantially lowers resistive losses in the device.
Conventional motors and alternators use varying electrical current within the windings to create a varying magnetic field in either the stator the rotor or both. Embodiments of the present invention instead vary a constant magnetic field by altering the flux path of said magnetic field. Hysteresis and eddy currents are the main sources of core loss. Hysteresis is caused by the reversal of magnetic polarity in a material and eddy currents are caused by change of magnetic field strength in a material whether or not the field reverses. Embodiments of the present invention achieve low core losses by preventing hysteresis in the bulk of the core and for this part of the core using a material resistant to eddy currents, such as powdered iron. Embodiments of the present invention further reduce core losses by using a material subject to low hysteresis losses in the small fraction of the core that does experience a reversal of magnetic field.
The motor/alternator according to embodiments of the present invention uses a single magnet with a north pole and a south pole. The magnet optionally is a permanent magnet or an electromagnet or a combination of the two. A plurality of flux conductors direct the magnetic field of the single magnet. Half of the flux conductors are in contact with the north pole of the magnet so that they have positive polarity and half of the flux conductors are in contact with the south pole of the magnet so that they have negative polarity. The north and south flux conductors are separated from each other by an air gap that is sufficient to minimize flux leakage between the conductors. A plurality of switch devices are attached to a rotor. The switch devices make contact between the flux conductors to alternately open and close a magnetic circuit for conducting magnetic flux between the north and south poles of the device""s magnet. These flux switches are the only part of the device undergoing hysteresis. The flux conductors are arranged so that they alternately conduct the magnetic flux in opposite directions around a power coil. Half of the flux conductors create a clockwise magnetic field around the power coil and the other half create a counter-clockwise magnetic field around the coil. As the switch devices open and close the alternating magnetic circuits, the polarity of the magnetic field surrounding the power coil is reversed. When used as an alternator, the reversal of the polarity of the magnetic field induces an alternating EMF voltage in the power coil. If an AC voltage is applied to the power coil, then the device will act as a motor by causing the switch devices on the rotor to move between flux conductors.
The flux conductors can be very small so many pairs can fit within a small space before they become too close and magnetic leakage occurs. Since each pair of flux conductors consists of one pole in the motor/alternator, many times more poles are possible with embodiments of the present invention than with prior art motors and alternators. The high number of poles allows embodiments of the present invention to achieve the high electrical frequency required for a high power density while running at modest rotational speed.
A device according to embodiments of the invention has the advantage that it uses only a single magnet. This allows for very simple and economical construction compared to many prior art motors and alternators. Embodiments of the present invention result in a motor or alternator with a large number of poles that does not require a large number of magnets. Furthermore since the magnet is on the stator there is no need for slip rings to bring electric current to the electromagnet, greatly simplifying its implementation. Since the magnet optionally is either a permanent magnet or an electromagnet, there is great flexibility in selecting the magnet that works best for the desired use. One possibility is that the magnet is a hybrid permanent magnet, electromagnet combination. In this arrangement a permanent magnet provides a fixed strength magnetic field and an additional electromagnet is used to augment the field to strengthen it or potentially to reduce it. By adjusting the strength of the field from the electromagnet, the total field strength of the motor or alternator is optionally adjusted as necessary.
An alternate embodiment of the present invention provides a three phase device. In its three phase embodiment still only one magnet or hybrid magnet is needed which magnetizes flux conductors which surround three separate coils arrayed one above another. Flux switches are arrayed such that a magnetic circuit is completed around one coil at a time. The flux switches are spaced such that three phase output is created when used as an alternator, and such that three phase power drives the device when used as a motor.
Embodiments of the present invention have many possibilities in layout and geometry. The rotor optionally is on the inside or the outside of the stator, or even on the face. The flux switches and flux conductors optionally take a variety of shapes depending e.g. on the intended use. The magnet optionally is permanent, electric, or both. There are still more variations possible not described here but well within the scope of the invention.
The motor/alternator according to embodiments of the present invention operates at very high electrical frequency for a given rotational speed compared to prior art devices. This results in very high power density. In one embodiment, the operating electrical frequency is ten times higher than prior art devices for a given rotational speed. This results in a power density that is ten times higher. The high frequency operation also results in decreased need for capacitors to smooth the power output when the device is used as an alternator. The high frequency operation further allows the device to operate at much higher voltage compared to prior art devices, thereby improving the battery charging capability of the device or simplifying its interface with an inverter. The higher voltage also results in smaller wires in the device, lower current, and lower power losses.
Additional features and advantages according to the invention in its various embodiments will be apparent from the remainder of this disclosure.